Recent years have seen a rapid expansion in the performance of solid state lighting emitters such as light emitting devices (LEDs); and with improved performance, there has been an attendant expansion in the variety of applications for such light sources. For example, rapid improvements in semiconductors and related manufacturing technologies are driving a trend in the lighting industry toward the use of light emitting diodes (LEDs) or other solid state light sources to produce light for general lighting applications to meet the need for more efficient lighting technologies and to address ever increasing costs of energy along with concerns about global warming due to consumption of fossil fuels to generate energy. LED solutions also are more environmentally friendly than competing technologies, such as compact fluorescent lamps, for replacements for traditional incandescent lamps.
Many solid state lighting systems have offered dynamic color control, however, the adjustment or tuning of the color of the output light has relied on LED centric approaches. Many solid state light sources produce light of specific limited spectral characteristics. To obtain light of a different color one approach uses solid sources that produce light of two or more different colors or wavelengths and one or more optical processing elements to combine or mix the light of the various wavelengths to produce the desired characteristic in the output light. One technique involves mixing or combining individual light from LEDs of three or more different wavelengths (spectral colors such as “primary” colors), for example from Red (R), Green (G) and Blue (B) LEDs. By selecting the relative intensities of the different sources, for example the amounts of red, green and blue light from the sources added to form the combined output light, it is possible to selectively produce light colors over a fairly substantial range of the visible spectrum.
With a LED-centric approach such as LED based RGB, the individual color amounts can be adjusted easily. However, using almost monochromatic colors from LEDs as the sources imposes limitations on overall performance. For example, with the approach using LEDs of three different very monochromatic colors, the output spectrum tends to have a small number of narrow spikes. For white light settings, this produces a low color rendering index (CRI) or otherwise inferior illumination of objects of certain colors where there is a gap in the output spectrum. Even for non-white settings, it may be difficult to produce an actual color output that really matches the desired spectral content because of the narrow spectrums combined to form the output, so that objects may take on unexpected hues when illuminated by such light outputs. It is possible to improve the light quality by providing additional LEDs of different colors, but that approach increases complexity and overall system cost and may still not achieve a desired high quality of light over a sufficient portion of the intended operating gamut.
There also have been a variety of proposals to enhance solid state light performance by adding appropriate phosphors to effectively shift some or all of the energy from the solid state source to a more desirable region of the spectrum. Although many such techniques are intended to provide white lighting, some have involved control, for example, to achieve and maintain a desired color characteristic of the white output light. For example, phosphor based techniques for generating white light from LEDs, currently favored by LED manufacturers, include UV or Blue LED pumped phosphors. In addition to traditional phosphors, semiconductor nanophosphors have been used more recently. The phosphor materials may be provided as part of the LED package (on or in close proximity to the actual semiconductor chip), or the phosphor materials may be provided remotely (e.g. on or in association with a macro optical processing element such as a diffuser or reflector outside the LED package). However, the tuning of the color or color temperature of the phosphor based approaches has still relied on dynamic control of different color LED sources. The phosphor emissions may be somewhat broader than those of the LEDs, and for example, might appear pastel or even substantially white. The controlled LEDs used for tuning may be specific color LEDs of one or more colors selected to adjust the light color characteristic of light produced by pumping of the phosphor. Like the LED-centric tuning of the substantially monochromatic LEDs, the LED centric tuning of the phosphor emissions may have some narrow spiking in the emission spectrum, and as a result, the range and quality of light color may still be less than desirable.
Solid state lighting technologies have advanced considerably in recent years, and such advances have encompassed any number of actual LED based products, however there is still room for further improvement in the context of lighting products. For example, it may be desirable for the solid state lighting device to provide a tunable color light output of color, however, there is still room for improvements in the range and/or quality of the tunable color light output. Relatively acceptable/pleasing form factors similar to those of well accepted lighting products also may be desirable, while maintaining advantages of solid state lighting, such as relatively high dependability, long life and efficient electrical drive of the solid state light emitters.